BYPASSING OF THE AQUEOUS PHASE BY METHANE GAS PRODUCED DURING CRUDE OIL BIODEGRADATION

Biodegradation of crude oil contamination in the subsurface has been studied for the last 30 years at a 1979 spill site located near Bemidji, Minnesota, USA. A separate phase oil body with oil saturations of 20-70% is centered on the water table 6-8 m below the surface. Degradation of the n-alkane fraction of the oil occurs by fermentation coupled to methanogenesis. The produced CH₄ and CO₂ migrate away from the oil in both the gas and aqueous phases. The CH₄ in the gas phase is oxidized to CO₂ in the vadose zone before reaching the land surface. Depleted N₂ concentrations in wells downgradient from the oil body indicate outgassing of CH₄ has occurred from the aqueous phase. However, dissolved N₂ is not completely depleted, which provides bounds on the amount of CH₄ cycled through the aqueous phase.

A two-dimensional reactive transport model of oil degradation, production of CH₄ and CO₂, and outgassing of CH₄ and CO₂ from the aqueous phase was constrained by CO₂ efflux data at the land surface and dissolved concentrations of CO₂, CH₄, and N₂. The results predict that 86% of observed surface efflux consists of outgassed carbon that could not have cycled through the aqueous phase sampled in the monitoring wells. The separation of the effect of directly outgassed CH₄ from the chemistry of the well samples suggests that there are isolated water pockets in contact with the degrading oil. These water pockets may be isolated by gas bubbles present in the oil body, which lower the relative water permeability, limiting the transport of water away from the oil. Outgassing from the aqueous phase is mostly in the form of CH₄, which is consistent with CH₄’s much lower solubility compared to CO₂. Preferential outgassing of CH₄ maintains low CH₄ to CO₂ aqueous concentration ratios, even when the plume is strongly methanogenic.